[rust] Clean up HAVE/USE RUST configuration

Cleanly separate enablement of Rust support from a `USE_RUST` that can
be used when invoking make, and `HAVE_RUST` that is defined internally
when rust support is enabled. This allows targets to decide for
themselves if Rust support is needed, or if it is optional.

The definitive decision about rust support will be determined if a
platform makefile includes `lib/rust_support/rules.mk` to include Rust
support in the build. It can optional do this based on a use-defined
`USE_RUST` flag.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

dev/uart/pl011/uart.c

@@ -8,7 +8,7 @@
 #include "dev/uart/pl011.h"
 
 /* If rust is in use, use the Rust implementation of this driver. */
-#if !USE_RUST
+#if !HAVE_RUST
 
 #include <assert.h>
 #include <dev/uart.h>
@@ -231,4 +231,4 @@ void uart_flush_rx(int port) {
 void uart_init_port(int port, uint baud) {
 }
 
-#endif /* not USE_RUST */
+#endif /* not HAVE_RUST */

lib/rust_support/rules.mk

@@ -25,10 +25,13 @@ MODULE_SRCS := \
 	       $(MODULE_SRCDIR)/src/lib.rs
 MODULE_OBJS := $(foreach d,$(MODULE_SRCS),$(call TOBUILDDIR,$(d)))
 
-# The platform must set RUST_TARGET and RUST_TARGET_PATH. If RUST_TARGET is a
-# standard tuple, RUST_TARGET_PATH should have the same value. Otherwise,
-# RUST_TARGET_PATH should point to the custom target json file, and RUST_TARGET
-# should be the basename of the json file.
+# USE_RUST can be used by platforms to allow invocation to determine if rust
+# support is present. However, when this module is included, we have rust
+# support. The define `HAVE_RUST` will indicate that rust is actually being
+# used.
+HAVE_RUST := 1
+
+GLOBAL_DEFINES += HAVE_RUST=$(HAVE_RUST)
 
 ifeq ($(RUST_TARGET),)
 $(error RUST_TARGET is not set for rust support)

platform/qemu-virt-arm/platform.c

@@ -87,7 +87,7 @@ void platform_early_init(void) {
     fdtwalk_setup_memory(fdt, MEMORY_BASE_PHYS, MEMORY_BASE_PHYS, DEFAULT_MEMORY_SIZE);
 }
 
-#if USE_RUST
+#if HAVE_RUST
 void setup_rust(void);
 #else
 static void setup_rust(void) {}

platform/qemu-virt-arm/rules.mk

@@ -24,8 +24,6 @@ MEMBASE := 0x40000000
 MEMSIZE ?= 0x08000000   # 512MB
 KERNEL_LOAD_OFFSET := 0x100000 # 1MB
 
-USE_RUST ?= 0
-
 MODULE_DEPS += \
     dev/bus/pci \
     dev/bus/pci/drivers \
@@ -41,6 +39,8 @@ MODULE_DEPS += \
     lib/fdtwalk \
     lib/fs/9p \
 
+USE_RUST ?= 0
+
 ifeq ($(USE_RUST),1)
 MODULE_DEPS += lib/rust_support
 endif
@@ -50,8 +50,7 @@ GLOBAL_DEFINES += \
     MEMSIZE=$(MEMSIZE) \
     PLATFORM_SUPPORTS_PANIC_SHELL=1 \
     CONSOLE_HAS_INPUT_BUFFER=1 \
-    TIMER_ARM_GENERIC_SELECTED=CNTV \
-    USE_RUST=$(USE_RUST)
+    TIMER_ARM_GENERIC_SELECTED=CNTV
 
 GLOBAL_DEFINES += MMU_WITH_TRAMPOLINE=1